The invention relates to an inline method for producing a spring strip profile for a slatted frame which comprises at least one core strand formed by a fiber-reinforced plastic and at least one thermoplastic cover layer surrounding the core strand. The method comprises at least the following steps: joining multiple fibers, threads and/or filaments to form a fiber bundle; impregnating the fiber bundle with a thermally activatable reaction resin; molding the outer contour of the fiber bundle impregnated with the reaction resin; thermally activating the reaction resin to form a cured core strand; introducing the core strand into an extruder head; applying thermoplastic melt in the extruder head to form the cover layer on the spring strip profile; and cooling and calibrating the spring strip profile in a cooling and calibrating device.
EP 2 865 296 A1 shows various profile shapes for a spring strip profile, which comprises a core strand made of a fiber-reinforced plastic, in particular a glass-fiber-reinforced plastic, and a thermoplastic cover layer as a sheath. The advantage of such a spring strip profile over the spring strips made of wood usually used to form slatted frames is mainly that no pre-bending of the strip, a so-called segmental arch is required. In fact, with the composite profile, the same carrying capacities and a consistent, permanent elasticity can be achieved with a straight profile section, which can be cut to any length and is easier to handle and storable than curved spring strips. The production of such a composite profile is usually carried out by having the core strand produced first and stored on rolls, until it is unwound again and coated by extrusion with the thermoplastic cover layer. Interim storage is required because the throughput speeds in the so-called “pultrusion” process used to make the core rod are many times lower than the take-off speed during the extrusion. The winding of a glass fiber-reinforced core rod requires considerable forces and is no longer possible beginning at a certain cross-sectional diameter of the core rod. Also, the later use of the tensioned winding roll in the extrusion line poses safety risks should the core profile bar that is under high bending stress break. A solid cross-section for the core profile is essential because a tubular structure buckles and breaks during winding. However, particularly the necessary interim storage of the prefabricated core profile bar on rolls leads to a significant loss of time, which entails economic disadvantages in the production of the spring strip.
Known from FR 2 715 280 is a generic inline method for forming a spring strip from a composite profile, which avoids the disadvantages associated with the interim storage. Here, a spring strip is produced in a single operational step by initially forming the glass-fiber-reinforced core strand using an in principle also known pultrusion method and then directly following, in the same production line, extruding the thermoplastic cover layer onto it. The slatted profile finished completely inline can be cut to the required length and packed at the end of the production line. The disadvantage of this known inline method for producing a spring strip profile is that the throughput speed through the pultrusion process is very slow, i.e., the theoretically possible throughput speeds in the downstream extrusion cannot be utilized, because a certain residence time of the glass-fiber bundle impregnated with a reaction resin in the pultrusion tool is required to effect sufficient curing. If curing has not progressed sufficiently, the cross-section of the core profile bar is changed by the pressure of the thermoplastic melt or the structure of the core profile is even destroyed during the subsequent extrusion. A higher heat input per unit of time can also not be achieved with the known method by arbitrarily extending the heated pultrusion tool because the frictional forces in the tool would then no longer be handled with a reasonable expenditure.